1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for performing a marine seismic survey using underwater nodes that carry appropriate seismic sensors.
2. Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of a geophysical structure under the seafloor. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. Thus, providing a high-resolution image of geophysical structures under the seafloor is an ongoing process.
Reflection seismology is a method of geophysical exploration to determine the properties of earth's subsurface, which is especially helpful in the oil and gas industry. Marine reflection seismology is based on using a controlled source of energy that sends the energy into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to evaluate the depth of features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
A traditional system for generating seismic waves and recording their reflections off geological structures present in the subsurface is illustrated in FIG. 1. A vessel 10 tows an array of seismic receivers 11 provided on streamers 12. The streamers may be disposed horizontally, i.e., lying at a constant depth relative to the ocean surface 14. The streamers may be disposed to have other than horizontal spatial arrangements. The vessel 10 also tows a seismic source array 16 configured to generate a seismic wave 18. The seismic wave 18 propagates downward toward the seafloor 20 and penetrates the seafloor until eventually a reflecting structure 22 (reflector) reflects the seismic wave. The reflected seismic wave 24 propagates upward until it is detected by the receiver 11 on the streamer 12. Based on the data collected by the receiver 11, an image of the subsurface is generated by further analyses of the collected data. The seismic source array 16 includes plural individual source elements.
However, this traditional configuration is expensive because the cost of the streamers is high. In addition, this configuration might not provide accurate results because coupling between seismic receivers and the sea water is poor for s-waves. To overcome this last problem, new technologies deploy plural seismic sensors on the bottom of the ocean to improve the coupling.
One such new technology is ocean bottom station (OBS) nodes. OBSs are capable of providing better data than conventional acquisition systems because of their wide-azimuth geometry. Wide-azimuth coverage is helpful for imaging beneath complex overburdens such as those associated with salt bodies. Salt bodies act like huge lenses, distorting seismic waves propagating through them. To image subsalt targets, it is preferable to have the capability to image through complex overburdens, but even the best imaging technology alone is not enough. Good illumination of the targets is necessary. Conventional streamer surveys are operated with a single seismic vessel and have narrow azimuthal coverage. If either the source or the receiver is located above an overburden anomaly, the illumination of some targets is likely to be poor. OBS nodes can achieve wide-azimuth geometry and solve this problem.
Additionally, OBS nodes are more practical in the presence of obstacles such as production facilities. For the purpose of seismic monitoring with repeat surveys (4D), OBSs have better positioning repeatability than streamers. Also, OBSs provide multi-component data that can separate up- and down-going waves at the seabed, which is useful for multiple attenuations and for imaging using the multiples. In addition, multi-component data allows for recording shear waves, which provides additional information about lithology and fractures, and sometimes allows for imaging targets that have low reflectivity or which are under gas clouds.
U.S. Pat. No. 6,932,185, the entire content of which is incorporated herein by reference, discloses an OBS. In this case, the seismic sensors 60 are attached, as shown in FIG. 2 (which corresponds to FIG. 4 of the patent), to a heavy pedestal 62. A station 64 that includes the sensors 60 is launched from a vessel and arrives, due to its gravity, at a desired position. The station 64 remains on the ocean bottom permanently. Data recorded by sensors 60 is transferred through a cable 66 to a mobile station 68. When necessary, the mobile station 68 may be brought to the surface to retrieve the data.
Although this method provides a better coupling between the seabed and the sensors, the method is still expensive and not flexible because the stations and corresponding sensors are left on the seabed.
An improvement to this method is described, for example, in European Patent No. EP 1 217 390, the entire content of which is incorporated herein by reference. In this document, a sensor 70 (see FIG. 3) is removably attached to a pedestal 72 together with a memory device 74. After recording the seismic waves, the sensor 70 and memory device 74 are instructed by a vessel 76 to detach from the pedestal 72 and rise to the ocean surface 78 to be picked up by the vessel 76.
However, this configuration is not very reliable because the mechanism maintaining the sensor 70 connected to the pedestal 72 may fail to release the sensor 70. Also, the sensor 70 and pedestal 72 may not reach their intended positions on the ocean bottom. Further, leaving the pedestals 72 behind contributes to ocean pollution and increases survey cost, which are both undesirable.
A further improved autonomous ocean bottom node seismic recording device (Trilobit node disclosed in U.S. Pat. No. 7,646,670, the entire content of which is incorporated herein by reference) having an integrated modular design and one or more features that assist coupling of the unit to the seafloor and improve the azimuthal fidelity of seismic signal measurement (vector fidelity) has been developed by the assignee of the present patent application. An example of a Trilobit node 400 is shown in FIG. 4. The node 400 has a base plate 402 holding various components, including a signal recording unit housing 404 and two battery housings 406. A hydrophone 408 is positioned in the center of the vector sensor housing 410. The vector sensor housing may also include geophones. Also shown in FIG. 4 is a handle 412 which allows for removal of the recording unit from the device, a clamp 414 which allows for securing of the recording unit when installed in the device, and a fixed connector 416, located at the rear of the signal recording unit housing, to allow for a communication connection between the recording unit and other components of the device.
However, even this node has its own limitations, e.g., the node needs to be returned to the vessel for the data to be removed, and the batteries need to be charged. Because the batteries are fixedly attached to the base plate, the charging process may take a number of hours, during which time the node cannot be used.
Accordingly, it would be desirable to provide systems and methods that provide a marine node for recording seismic waves that can be retrieved on the vessel and readied for a next deployment in a short period of time.